Harvested porcine mitral xenograft fixation: impact on fluid dynamic performance.

BACKGROUND AND AIM OF THE STUDY: Recent developments suggest that stentless bioprosthetic mitral valve heterografts should be considered in order to optimize valve hydrodynamics. The fixation process alters the mechanical properties of tissue. This study investigates the changes in mitral valve morphology and hemodynamic performance following fixation. METHODS: Porcine mitral valves were excised and attached to a physiological annular ring. Mitral valve function was studied in vitro with a rigid transparent left heart model, allowing transverse and sagittal views. Initial experiments were performed with fresh valves under physiological conditions. Three different papillary muscle positions were used, and each was recorded. After glutaraldehyde fixation, genipin fixation, and cryopreservation, the valves were re-studied while maintaining cardiac output. Performance characteristics before and after fixation were obtained from hydrodynamic pressure and flow data, high-speed video camera, digital video, Doppler ultrasound, and three-dimensional papillary muscle force measurements. Morphology changes were detected by detailed anatomic measurements of the valves before and after fixation. RESULTS: Valve length was reduced by 18.5% after fixation with genipin (p <0.001), but not with glutaraldehyde. Cryopreserved valves showed no statistically significant changes in morphology or hydrodynamic performance after preservation. The forward flow opening area was reduced by 12.2% (p <0.001) after glutaraldehyde fixation, and by 32.3% (p = 0.004) after genipin fixation. Thus, maximal forward flow velocity was increased by 33.3% (p = 0.008) after glutaraldehyde fixation and by 52.8% (p = 0.001) after genipin fixation. The flow acceleration was consistent with a funnel shape of the fixed valves causing important flow contraction beyond the orifice (vena contracta). The papillary muscle force increased with apically posterior papillary muscle displacement by 20.4% (p = 0.001) and 101.5% (p <0.001) after glutaraldehyde and genipin fixation, respectively, and total regurgitant volume was increased by 91.6% (p <0.001) and 117.3% (p <0.001), respectively. The work required by the heart simulator to maintain a constant cardiac output at constant vascular resistance increased by 24.2% (p = 0.003) and 34.2% (p = 0.004) after glutaraldehyde and genipin fixation, respectively. CONCLUSION: The present study shows that chemical fixation of porcine mitral valves adversely affects the hemodynamics of the valves, increasing overall workload. The effects were more severe after fixation with genipin than with glutaraldehyde. This suggests the need to explore other fixation agents to optimize valvular cardiac function. Cryopreservation had no detrimental effects on valvular hemodynamic performance.

Time variability in Cenozoic reconstructions of mantle heat flow: plate tectonic cycles and implications for Earth's thermal evolution.

The thermal evolution of Earth is governed by the rate of secular cooling and the amount of radiogenic heating. If mantle heat sources are known, surface heat flow at different times may be used to deduce the efficiency of convective cooling and ultimately the temporal character of plate tectonics. We estimate global heat flow from 65 Ma to the present using seafloor age reconstructions and a modified half-space cooling model, and we find that heat flow has decreased by approximately 0.15% every million years during the Cenozoic. By examining geometric trends in plate reconstructions since 120 Ma, we show that the reduction in heat flow is due to a decrease in the area of ridge-proximal oceanic crust. Even accounting for uncertainties in plate reconstructions, the rate of heat flow decrease is an order of magnitude faster than estimates based on smooth, parameterized cooling models. This implies that heat flow experiences short-term fluctuations associated with plate tectonic cyclicity. Continental separation does not appear to directly control convective wavelengths, but rather indirectly affects how oceanic plate systems adjust to accommodate global heat transport. Given that today's heat flow may be unusually low, secular cooling rates estimated from present-day values will tend to underestimate the average cooling rate. Thus, a mechanism that causes less efficient tectonic heat transport at higher temperatures may be required to prevent an unreasonably hot mantle in the recent past.